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A Long Island man has been accused of drowning his mother in the pool at their home in an affluent neighborhood on the north shore before dumping her body by a shed and going to Manhattan to confess to his sister, according to prosecutors. Greg Cergol reports. (Published Thursday, Aug. 18, 2016)

A Long Island man has been accused of drowning his mother in the pool at their home in an affluent neighborhood on the north shore before dumping her body by a shed and going to Manhattan to confess to his sister, according to prosecutors.

Dennis Cullen Jr., 23, was arrested Wednesday evening on charges including murder in the death of his mother, 63-year-old Elizabeth Cullen.

At the man's arraignment on Thursday, prosecutors alleged that Dennis Cullen had been arguing with his mother at their home on White Hill Road in Lloyd harbor because he hadn't been taking his medication. At one point, she poked him, and the 23-year-old dragged his 5-foot-2-inch mother to the pool and allegedly held her underwater as she fought to break free.

"He put her in a headlock, walked from the shallow end of the pool to the dep end, all the time with her struggling," said Robert Biancavilla, a homicide prosecutor with the Suffolk County DA's office.

Prosecutors said that in a videotaped confession to police, Dennis Cullen allegedly told investigators that he was surprised by his mother's struggle, but dumped her body by a shed and covered it with a dinghy once she died.

After the killing, prosecutors said that the man took money and credit cards from his mother's wallet and drove her car to the LIRR station in nearby Cold Spring Harbor. He got on a train to Manhattan, and went to one of his sisters' home to allegedly tell her about what had happened.

Elizabeth Cullen's body was discovered later that evening after someone called police to check on the woman. It's not clear if it was Dennis Cullen's sister who made the call.

Homicide detectives descended on Cullen's neighborhood — an area where crime is rare and multi-million dollar homes dot the landscape.

Cullen does not have a criminal history, and authorities would not indicate what his medication was for. He is being held on $5 million cash bail or $8 million bond. Neither Cullen's father nor any of his three sisters appeared in court.

Family friends said that the 23-year-old had a history of mental illness, however.

His court-appointed attorney, Steve Fondulis, said his client was not guilty.

"I spoke to him for a couple of minutes. He's pretty calm and as I indicated, he hasn't been convicted of anything so he's not sorry -- he hasn't said he's sorry," Fondulis said.

Elizabeth Cullen was an interior designer who one friend said worked out of her home. A source said she was the daughter of U.S. Army major general George Casey, killed in Vietnam in 1970. Her brother served as U.S. Army chief of staff as recently as 2011.